It is known that frequency synchronism must exist between the oscillator and the input circuit of a superheterodyne receiver.
In order to attain the required synchronism between oscillator and input circuit, various techniques are employed. For example, it can be attempted to achieve the desired synchronism by specially cutting the discs of the rotary tuning capacitor. However, for electronic tuning systems varactor diodes which have specially adapted capacitance/voltage characteristics are not available. For this reason, tuning systems with varactor diodes employ the known three-point tracking which, however, permits optimum tracking, or synchronization only at three points of the frequency range. Even with precisely identical characteristics of the tuning elements or diodes, there occur synchronization deviations which result in sensitivity breaks within the tuning range. Moreover, inequality of the characteristics and deviations in the capacitance value of the padding capacitor produce additional deviations and thus increase the problem.